1. Field of the Invention
This patent application is related to the field of signal processing. More particularly, the invention relates to the use of ultrasound Doppler measurements.
2. Related Art
Preeclampsia is a major cause of maternal mortality as well as perinatal morbidity and mortality. It is mainly characterized by hypertension and protienuria. Preeclampsia affects as many as one in ten of all pregnancies if its milder forms are counted. The incidence of preeclampsia is higher in women who are nulliparous, carry multiple fetuses, have chronic hypertension, diabetes, or if the woman has had previous preeclamptic pregnancies. Although clinical manifestations do not appear until the final three months, the foundations for preeclampsia are thought to be established in the first 10-12 weeks of gestation.
Maternal complications due to preeclampsia are associated with the vascular system. Mothers are at risk for intravascular coagulation, bleeding, and organ failure. Maternal blood volume is reduced and has altered distribution. The treatment of hypertension in pregnancy does not eliminate or reverse the process but does help prevent maternal cardiovascular complications, especially during labor and delivery.
Evidence exists to suggest that there is inadequate trophoblast secondary invasion in pregnancies that later developed preeclampsia. The adaptation to a limited supply of nutrients due to inadequate secondary trophoblast invasion in preeclampsia may lead to damages in the structure and the metabolism of a fetal life. Suboptimum supply of oxygen and nutrients may also lead to fetal intrauterine growth restriction. This, together with premature delivery, poses major threats to the fetus leading to various degrees of morbidity and even death. Long term follow up studies have demonstrated that babies who have suffered intrauterine growth retardation and those who are small and disproportionate at birth are more likely to develop hypertension, coronary heart disease and non-insulin-dependent diabetes in adult life. These observations suggest that fetuses are quite sensitive to the maternal placental function.
Various methods have been developed to detect preeclamptic pregnancies early in the gestational period. The use of ambulatory pressure monitoring for identifying hypertensive pregnancies has been used. Using this method, blood pressure monitoring can be performed in four-week intervals for 48 hours throughout gestation starting in the 16th week. Ambulatory pressure monitoring has shown significant results with specificity and sensitivity above 90%. However, the instrumentation required for pressure monitoring is expensive and reported lower acceptability and sleep disturbances among patients.
Doppler ultrasound blood velocity waveforms have also been employed for predicting preeclampsia. A uterine arterial pulsatility index (PI) has been studied as an indicator to the increased resistance to the placental flow during the 11th through 14th week and 23rd week of gestation in various studies using Doppler ultrasound measurements. Analysis of the Doppler ultrasound measurements has proven difficult, however. Current techniques to analyze the measurements have resulted in sensitivities of mean PI for preeclampsia at rates as low as 25-27% during 1-14 weeks and 41% at 23 weeks. Superior techniques for analyzing data from Doppler ultrasound measurements are greatly desired.